US Patent 8171800 Differential pressure sensor using dual backside absolute pressure sensing

A MEMS differential pressure sensing element is provided by two separate silicon dies attached to opposite sides of a silicon or glass spacer, the sides of which are recessed and the recesses formed therein at least partially evacuated. The dies are attached to the spacer using silicon-to-silicon bonding provided in part by silicon oxide layers if a silicon spacer is used. The dies can be also attached to the spacer using anodic bonding if a glass spacer is used. Conductive vias extend through the layers and provide electrical connections between Wheatstone bridge circuits formed from piezoresistors in the silicon dies.